Pollsters quizzed 1,001 adults Friday through Sunday for the poll; most questions had a sampling error of plus or minus 3 percentage points.

Fifty-seven percent said they believe the March 2003 invasion of Iraq was a mistake, near September's record high of 59 percent. That question had a sampling error of plus or minus 4.5 points.(Interactive: Poll results)

Bush's approval rating of 36 percent is the lowest mark of his presidency in a Gallup poll, falling a percentage point below the 37 percent approval he scored in November. The previous CNN/USA Today/Gallup poll, conducted February 28-March 1, put his job approval at 38 percent. (View Bush's second term approval ratings)

Sixty percent of those surveyed in the latest poll said they disapproved of his performance in office, the same figure as in the last poll. (Read full results document -- PDF)

Certain about Iraq

The poll found Bush's fortunes are tied to Iraq, where more than 2,300 U.S. troops have been killed.

Two-thirds of those surveyed told pollsters that history will remember Bush most for the March 2003 invasion that toppled Iraqi leader Saddam Hussein and the battle against a persistent insurgency that followed the Hussein regime's collapse.

Bush launched his latest effort to shore up support for the war Monday, accusing Iran of providing explosives used to attack American troops and telling an audience at George Washington University that U.S. forces were "making progress" against insurgents.

He also praised Iraqis for averting civil war despite the sectarian violence that came after February's bombing of the al-Askariya mosque in Samarra, a revered Shiite Muslim shrine.

"The situation in Iraq is still tense, and we're still seeing acts of sectarian violence and reprisal," Bush said. "Yet out of this crisis, we've also seen signs of a hopeful future." (Full story)

With congressional elections approaching, public discontent with the war appeared to be taking a toll on Bush's fellow Republicans.

Only 32 percent polled over the weekend said they thought Bush had a clear plan for handling the situation in Iraq, while 67 percent said he did not.

Only 25 percent said Democrats had a clear plan -- but 48 percent said Democrats would do a better job managing the issue, while 40 percent favored Republicans.

Democrats enjoy lead

Those figures, along with weakened support for GOP handling of the battle against terrorism, have given Democrats a 16 percentage point lead over Republicans when registered voters are asked which party they will support in November.

Democrats drew the support of 55 percent of the registered voters questioned, while 39 percent said they would be voting Republican in the fall. That question had a sampling error of plus or minus 4.5 percentage points.

Republicans held a 4-point advantage over Democrats on dealing with terrorism, 45 to 41 percent. And despite increasing optimism about economic conditions, Democrats held a strong lead over the GOP, 53-38 percent, when asked which party would better manage the economy.

To make the case for war, Bush and other top officials said the invasion of Iraq was necessary to strip the country of illicit stockpiles of weapons of mass destruction. U.S. inspectors later concluded that Iraq had dismantled its weapons programs under U.N. sanctions in the 1990s, though it had concealed some weapons-related research from the United Nations.

The latest poll found 51 percent of Americans believed the administration deliberately misled the public about whether Iraq had weapons of mass destruction, while 46 percent disagreed. That question had a sampling error of 4.5 percentage points as well.